Lisinopril, as shown in FIG. 1, is a lysine derivate of Enalapril, and belongs to the third-generation long-acting angiotensin-converting enzyme inhibitor (ACEI). Lisinopril inhibits the renin-angiotensin-aldosterone system to reduce the blood pressure, and at the same time, it also has an antihypertension effect on low-rennin hypertension. Lisinopril has the advantages that the acting duration is long, the trough-to-peak ratio of reduced systolic pressure and diastolic pressure is high, the antihypertension effect is stable, and no liver lesion is caused because Lisinopril is not converted by liver after oral administration and absorption, so Lisinopril is superior to other drugs for patients with liver diseases and hepatic dysfunction. Presently, Lisinopril has become one of drugs of choice for treatment of hypertension.
N2-[1-(S)-ethoxycarbonyl-3-phenylpropyl]-N6-trifluoroacetyl-L-lysine is a key intermediate for synthesis of Lisinopril, and has a structural formula shown in FIG. 2. Michael addition of N6-trifluoroacetyl-L-lysine to β-ethyl benzoylacrylate followed by catalytic hydrogenation, provides a crude mixture of N2-[1-ethoxycarbonyl-3-phenylpropyl]-N6-trifluoroacetyl-L-lysine with a diastereomeric ratio of 80:20 SS/RS, respectively. Patents of JP1986/68970, JP1991/22867, JP1992/4308, JP1994/336495 and WO1997/043246 disclose processes for preparations of similar compounds. The 1R-isomer is the main impurity, and has a structural formula shown in FIG. 3. WO1997/043246 does not disclose any method for purifying and isolating the 1S-isomer, and does not disclose a method for removing the undesired 1R-isomer. The crude products are directly applied to the subsequent reaction, and the yield of the final product is lower, and at the same time, the cost of the raw material and the production cost are increased. JP1986/68970 and JP1992/4308 disclose a recrystallization method using water and ethanol, but the yield is low. JP2003/026644 discloses a purification method of re crystallization using water with a water-soluble solvent such as acetonitrile or tetrahydrofuranbygradient cooling where 1R-isomer is removed. However, good solubility of N2-[1-ethoxycarbonyl-3-phenylpropyl]-N6-trifluoroacetyl-L-lysine in acetonitrile or tetrahydrofuran results in lower yield of recrystallization, and it becomes more obvious on large quantity production. Additionally, it is difficult to control gradient cooling on manufacturing scale, and acetonitrile and tetrahydrofuran are relatively expensive solvents.